The present invention is directed to a method and apparatus for the discrimination between liquid and air in a segmented flowing stream, such as one found in analytical instrumentation, e.g., a clinical chemistry analyzer.
Devices of this type, called bubble detectors in these systems, are known in the prior art and use dedicated light sources, such as light emitting diodes with relay optics such as lenses and apertures close to the liquid carrying tube to transmit light through the tube to a photodetector on the opposite side of the stream. Discrimination between liquid and air results from changes in the light concentration on the photodetector due to the difference in the refractive index of air and liquid within the tube. Because the light passes through the liquid on the way to the detector, variations in absorbance characteristics of the liquid can influence the intensity of light striking the photodetector, thereby affecting the ratio of air to liquid signal.
In other prior art optical bubble detectors, a separate light path is used adjacent to a colorimetric optical path, resulting in a bubble detection signal offset in time from the analytic signal. Because of this offset, timing compensation is required when taking data on segments of the stream entering the colorimetric optical path. This is especially problematic when the stream does not have a constant velocity and the time can vary from the bubble detector position to the colorimetric position of the tube.
Moreover, the signal level of these known detection systems rely on the focusing effect of the liquid within the tube to increase the signal level, while air in the tube reduces the signal level enabling discrimination between the two. The wavelength of the bubble detector light must be chosen to avoid absorption of the bubble detector light by the liquid within the tube and provisions must be made to prevent the bubble detector light from influencing the colorimetric measurement.